Line disappears in Acrobat when header is added

I've never encountered this before and I have been using Acrobat for years.  I have a document that was not created by me, so I do not know what it was created in, but it looks like they converted it directly to PDF.  When I go to add a header to the PDF, a whole line of text at the top part of the page disappears (It's 3 lines down, so the first two lines are there still, but the third line disappears).  It does not happen on every document that is converted to PDF.  What can be causing this?  (And I am not an IT person, so in layman's terms please!)  I use this quite often for documents, so this is kind of upsetting that I may have to find something else to use if I can't find a fix for this, as these are important documents.